1. Field of the Invention
This invention pertains to electrically conductive compositions in general, and specifically to thick film copper and glass containing conductives formulated for screen printing application on aluminum nitride substrates.
2. Description of the Related Art
Modern electronic circuits are almost exclusively assembled via printed wiring substrates. These substrates eliminate the need for the "rat's nest" of wires that abound with discretely wired circuitry and improve the yield of assembled circuitry because the wiring boards improve the consistency of device interconnection. Additionally, wiring boards have encouraged miniaturization by providing size benefit as well as signal integrity that might otherwise be compromised on higher frequency circuitry.
Wiring substrates for commercial applications are typically fabricated from a polymer material such as epoxy or polyimide which may be cast or laminated into the desired configuration. The polymer materials are desirable because of low cost and the relative ease of general processing but have drawbacks in thermal characteristics such as maximum exposure temperature, maximum operating temperature, thermal coefficient of expansion, and thermal conductivity. Polymer substrates also may suffer from poor aged performance, mechanical strength and rigidity and other factors.
Where polymer wiring substrates have proved to be ineffective in meeting the demands of an application, the electronics industry has generally relied upon either alumina, porcelain or glass, or beryllia substrates. These substrates typically have better performance in many of the areas that prevent the use of polymer substrates. However, as with most things in the real world, these substrates are not also without particular limitation. For example, beryllia is a very good thermal conductor having high mechanical strength. Unfortunately, beryllia is also very toxic if inhaled in dust form. Exposure to beryllia dust is now restricted in the work place in many countries around the world.
Aluminum nitride, only recently commercially viable, has many properties much more desirable than those heretofore known compositions. Aluminum nitride has thermal conductivity comparable to beryllia without the toxicity. Aluminum nitride has mechanical strength and chemical stability that is comparable to other ceramic substrates. Particularly advantageous is the thermal coefficient of expansion of this material, which very closely corresponds to the thermal coefficient of expansion of silicon, allowing for the mounting of semiconductor devices that are required to endure many thermal cycles without separating from the wiring substrate. When the high thermal conductivity of the material is considered in concert with a thermal coefficient of expansion matching silicon, the application to high power semiconductor circuits in small, high reliability packages becomes apparent.
In spite of the many advantages aluminum nitride has to offer, there are again some drawbacks. Principal among the heretofore limitations of aluminum nitride is the fact that the substrate is a non-oxide substrate. The use of a non-oxide substrate limits the selection of conductive and resistive materials suitable for application to the substrate and limits the method of application. Some of the formulations previously used with alumina react too vigorously with aluminum nitride or in other cases fail to generate any chemical adhesion whatsoever. For example, in the hybrid industry most circuits are formed upon alumina substrates using a thick film screen printing process. The screen printing process is favored where suitable over vapor deposition processes because of the lower costs of production involved.
However, the use of aluminum nitride has previously required either the use of precious metal formulations which are necessarily expensive and which are not good conductors, owing to the relatively poor intrinsic conductivity of the materials and the high price those precious metals are sold for on the open market, or the use of expensive vapor deposition processes using materials such as tungsten and molybdenum. The advantages offered by a copper-based screen printable conductive are well described in U.S. Pat. No. 4,186,615, incorporated herein by reference, which discusses the application of copper to oxide-based substrates. While heretofore known formulations were not found to be suitable for use on an aluminum nitride substrate, there is a place and a need for a copper conductor formed upon aluminum nitride.